1. Field of the Invention
This invention relates to semiconductor devices selectively coupling light from one waveguide to another for such applications as switching light between waveguides and attenuating or modulating light in a waveguide. More particularly, the present invention is directed to an optical switch in which light is resonant coupled between stacked, semiconductor waveguide layers grown on a substrate, separated by a coupling layer and bounded by containment layers with one waveguide layer being composed of a superlattice of multiple-quantum-well (MQW) material having an index of refraction which varies substantially with an applied electric field and the other waveguide layer composed of a material having an index of refraction which varies relatively little with the electric field. The electric field varies the index of refraction of the MQW waveguide layer between a value compatible with the index of refraction of the other waveguide to resonantly couple light between the waveguides, and a substantially different index of refraction in which no substantial amount of light is coupled between the waveguides.
2. Background Information
Semiconductor devices for switching light between waveguides are known. In one type of such devices the waveguides are formed side by side in a common plane with a suitable coupling material between. Typically, the index of refraction in the two waveguides is identical and that of the coupling material is lower so that there is resonant coupling between the waveguides in a cross propagation or "switch" condition. A "no switch" or parallel propagation condition is commonly created by an induced phase change in one of the guides, normally caused by an electric field induced change of the index of refraction (dn/dE) in one waveguide. The value of dn/dE is a factor which determines the magnitude of the electric field required to switch the light. The larger dn/dE, the smaller the voltage required for a given geometric configuration.
Other factors affecting the operation of such devices are the index of refraction and width of the coupling material. Within limits, the smaller the difference between the indices of refraction of the waveguides and the coupling material, and the narrower the width of the coupling material, the shorter is the length of the parallel waveguides required for cross coupling. In turn, a shorter device length results in decreased device capacitance, and thus an increase in the maximum switching speed and a decrease of the energy required per switching cycle.
Thus, material and fabrication constraints have set the performance and dimensional limits of such devices. Conventionally, the material used has been LiNbO.sub.3 or GaAs. Fabrication techniques, namely lithography and the need for well defined etching, usually set a lower limit of 3 micrometers for the width of the coupling material. This results in a required length of about 5 mm. Because of the relatively small dn/dE value for GaAs (7.10.sup.-10 cm/V), the required voltage for switching from cross-to-parallel propagation is on the order of 20 volts and the required energy is estimated to be about 240 pj. All of these values are too large for high level integration of the devices for complex applications.
It has been suggested in U.S. Pat. No. 4,048,591 that discrete waveguide elements can be stacked one on top of the other with a film of dielectric material in between, however, this patent concludes that it is preferable to place the two discrete waveguides side by side with a third coupling waveguide and the film of dielectric material overlapping both of them.
It has also been suggested in published United Kingdom patent application No. GB 2174212A that optical switching devices can be constructed from layers of semiconductor materials grown on the substrate. The waveguide layers are made of the same material and thus have identical indices of refraction. They are separated by a coupling layer having an index of refraction which varies substantially to effect switching between parallel and cross propagation. In these devices, the index of refraction of the coupling layer is controlled by current rather than voltage. The coupling layer is doped to provide the free carriers required to support the current. Such current operated devices require significantly more power than voltage operated devices.
Most of the optical switches utilize homogeneous materials such as LiNbO.sub.3, GaAs or ternary GaAlAs. U.S. Pat. No. 4,737,003 suggests the use of thin layers of multiple-quantum-well material as a selectively reflective layer at the intersection of two waveguides in a common plane. The index of refraction of the multiple-quantum-well reflective layer is varied through the injection of carriers to either reflect light into the intersecting waveguide or to let it pass straight through. This is a very complex device to construct, and again, using carrier injection to control the refraction index, it has a high power consumption.
Multiple-quantum-well structures comprise alternating very thin layers of materials having different conduction band energy levels to create quantum-wells between barrier layers. Such materials are known to exhibit an unusually strong dispersion of the index of refraction near excitonic transitions which are coupled to the electron-hole energies of the the well. However, the principal interest in these devices has been in the electro-absorptive effect or the change in absorption as a function of an electric field. Such devices have been investigated for use in external modulators for lasers used in transmitting data at high rates. The light generated by the laser is injected into the multiple-quantum-well material which is selected such that the light is absorbed in the "OFF" state and passed through in the "ON" state. One disadvantage of such a device is that there is still a great deal of absorption in the "ON" state so that the efficiency of the device is not favorable. In addition, the switching time of such devices is limited by the life time of the excited carriers.
There remains a need therefore for an optical coupler small in size suitable for large scale integration.
There is a concurrent need for such an optical coupler which operates at low voltages and with low energy consumption.
There is also a need for such an optical coupler having a very fast switching time.
There is a further need for such an optical coupler with improved separation between the on and off states.
There is yet another need for such an optical coupler with the above characteristics which can be used either as a light switch or an attenuator.
There is an additional need for such an attenuator exhibiting a reduction in residual absorption in the "ON" state.